Although Microsoft has stopped support for MS IE Web Controls, we would use it when we need some conrols like TreeView in .Net 1.1 - say sharepoint 2003
1. Add the controls in .ascx (used for web parts)
2. While deploying, copy the Microsoft.Web.UI.WebControls.dll to the bin of WSS root
3. Deploy the above dll in GAC (if required)
4. Run the Build.Bat file from C:\Program Files\IE Web Controls (installation folder). While doing this, care should be taken for CSC.exe location - either run it from VS.Net Command prompt (after switiching to the above location) or include the path of CSC.exe in the "Environment Variables" of system. This will generate a \build\Runtime folder with script file required for the controls
5. Copy the above folder to the root of WSS
6. Exclude the path for the above folder, say webctrl_client (only type the folder name as it is in the root folder) by configuring the virutal server (use SharePoint Central Administration page)
Now it will work!!